Guy mentioned that Mother Murk leads a coven of hags. That's what the wyresses told him, and that a Reach woman named Sibellec might know what to do with Mother Murk. She can be found at the northern enclave with the wyresses.

Follow the marker to find Sibellec in the camp. She has been waiting she says for them to turn up. It was written in the sky and the stones. Mother Murk's anger and darkness run deep, even for a hagraven, she leads the coven through fear and pain. To defeat her use this charm as she can siphon vitality from even the strongest foe. First kill the lesser hags and drain their power into this charm. However, only use the charm on Murk when she tries to siphon your life force. Don't waste the power.

Sibellec's Charm

The hags are to the west of here. Protect the charm as it's the best defense against Mother Murk. Head over to the bog and find some lesser hags to kill and use the charm to charge it. When four hags have been killed it's time to find Mother Murk.

Mother Murk is standing outside her hut, attack her and when she tries to use the vitality-draining power, use the charm to counter the effects. When she has been defeated head back to Sibellec in the wyress camp.

Sibellec is pleased and everything turned out as she had foreseen. She sensed Mother Murk's departure and wishes she will find peace in whatever lies beyond this plane. She was a dark and dangerous creature, to be sure, but she wishes for her to be at peace. With her gone now, the balance of the Fens will change. There will be no more danger for the refugees, the hags will wither, and the Wyrd will prosper.

She ends the conversation telling the Vestige they need to go as they have somewhere else to be. She see a long road ahead of them, tread carefully.